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CIE IGCSE PHYSICS Forces – Hookes Law

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CIE IGCSE PHYSICSForces – Hookes Law

23rd November 2011

Physics for IGCSE Physics pages 60-61

1.5 Effects of Forces

Core

• State that a force may produce a change in size and shape of a body

• Plot extension/load graphs and describe the associated experimental procedure

Supplement

• Interpret extension/load graphs

• State Hooke’s Law and recall and use the expression F = k x

F= Force k = spring constant x= extension length

• Recognise the significance of the term ‘limit of proportionality’ for an extension/load graph

Force can also change the shape of an object.

A stretching force puts an object such as a wire or spring under tension.

A squashing force puts an object under compression.

- Elastic materials return to their original shape when the forces on them are removed.
- Plastic materials retain their new shape, when forces are removed
- Hooke’s Law - The extension of a spring or wire is directly proportional to the force applied provided the limit of proportionality is not exceeded.
- Spring constant is the gradient of the force (x- axis) against extension (y-axis) the gradient is the spring constant Units N/m
- A stretching force puts an object such as a wire or spring under tension
- The limit of proportionality is the is the point beyond which Hooke’s law is no longer true when stretching a material.
- The elastic limit is the point beyond which the material you are stretching becomes permanently stretched so that the material does not return to its original length when the force is removed.
- A squashing force puts an object under compression.

Brittle materials such as glass do not change shape easily and break before noticeably stretching.

Resilient materials do not break easily.

Elastic materials return to their original shape when the forces on them are removed.

Plastic materials retain their new shape.

Experimental procedure:

1. Place the weight holder only on the spring and note the position of the pin against the metre rule.

2. Add 1N (100g) to the holder and note the new position of the pin.

3. Calculate the extension of the spring.

4. Repeat stages 1 to 3 for 2N, 3N, 4N, 5N and 6N. DO NOT EXCEED 6N.

spring

weights

pin

metre rule

Force (N)

0

Extension (mm)

0

Hooke’s law states that the extension of a spring force is proportional to the force used to stretch the spring.

‘Proportional’ means that if the force is doubled then the extension also doubles.

The line on a graph of force against extension will be a straight AND go through the origin.

Hooke’s Law

- When a material obeys Hooke’s Law, the force is proportional to the extension of the material.
- Force Extension
- F x
- This leads to the equation:
- F = kx
- Where k is a measure of the stiffness of the sample – the gradient of a force – extension graph.

Force-Extension Graphs

x

10

x

x

x

x

x

Force (N)

5

x

x

x

x

x

0

0

0.25

0.5

Extension (m)

Extension Work

- Sketch a force-extension graph (the line should be roughly halfway between your axes).
- Sketch a second line for a stiffer spring (harder to stretch).
- Sketch a third line for a less stiff spring (easier to stretch).
- Explain each line, relating the force to the extension.

Force-Extension Graphs

Force-Extension Graphs

Force-Extension Graphs

A spring of original length 150mm is extended by 30mm by a force of 4N. Calculate the length of the spring if a force of 12N is applied.

12N is three times 4N

Therefore the new extension should be 3 x 30mm

= 90mm

New spring length = 150mm + 90mm

= 240mm

The right hand spring has been stretched beyond its elastic limit

Up to a certain extension if the force is removed the spring will return to its original length. The spring is behaving elastically.

If this critical extension is exceeded, known as the elastic limit, the spring will be permanently stretched.

Hooke’s law is no longer obeyed by the spring if its elastic limit is exceeded.

Force (N)

elastic limit

0

Extension (mm)

0

Force

0

Extension

0

An elastic band does not obey Hooke’s law.

Choose appropriate words to fill in the gaps below:

Hooke’s law states that when a wire or spring is _________ the increase in length or _________ is proportional to the load ______ applied.

This law is not obeyed if the spring is taken beyond its ______ limit after which it will become _____________ stretched.

A ________ band does not obey Hooke’s law.

A graph illustrating Hooke’s law will have a line that is ___________ and passes through the _______.

stretched

extension

force

elastic

permanently

rubber

straight

origin

WORD SELECTION:

stretched

elastic

permanently

extension

origin

force

rubber

straight

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